2. Field of the Invention
The present invention generally relates to deposition of a metal film on a substrate. More particularly, the electric current/voltage established between an anode and a substrate seed layer during metal film deposition on a substrate.
3. Background of the Related Art
Electroplating, previously limited in integrated circuit design to the fabrication of lines on circuit boards, is now used to deposit metal film on substrates to form interconnect features, such as vias or trenches. One feature filling method that utilizes electroplating requires initially depositing a diffusion barrier layer on the substrate by a process such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or an electroless plating systems. A seed layer is deposited on the diffusion barrier layer by CVD, PVD, or an electroless plating systems to define a plating surface on the substrate. A metal film is then deposited on the substrate seed layer by electroplating. The deposited metal is planarized by another process such as chemical mechanical polishing (CMP), to define conductive interconnect features.
Electro-chemical plating (ECP), one embodiment of electroplating, is performed by establishing a voltage across an electrolyte solution between the anode and the substrate seed layer. Both the anode and the substrate seed layer are disposed in contact with the electrolyte solution during ECP. During normal ECP operations, a sufficient negative voltage, known as a plating voltage, is established between the seed layer on the substrate and the anode to attract metal ions in the electrolyte solution to deposit as metal film on the seed layer. The plating voltage is typically applied when the substrate seed layer is fully immersed in the electrolyte solution within the ECP system. A solid-state power supply, applied under the control of a controller, is used to apply electric voltages/currents between the anode and the seed layer. When electroplating substrates, efforts are made to ensure the production of a consistent electric current density across the seed layer on the substrate during the plating process. For each plating location on the seed layer, the metal film deposition rate varies as a function of the electric current density at that location. Therefore, variations in the electric current density across the seed layer on the substrate likely result in inconsistent plating rates and deposited metal film thickness across that seed layer.
When an ECP system is not being used to deposit metal film on a substrate for an extended period, such as during production downtime, portions of the head assembly such as the electric contacts are typically immersed in the electrolyte solution to limit oxidation of the head assembly that results from exposure of the electric contacts to air. Immersion of the head assembly into the electrolyte solution also minimizes formation of crystals that may form due to evaporation of water from the electrolyte solution containing chemicals such as copper sulfate.
Typical solid state power supplies for electroplating systems produce a slight or trickle current that flows from the anode to the electric contacts through the electrolyte solution even when the power supply is turned off. This slight or trickle current is known as a current leak, and the extent of the current leak is a quantified value in each solid state power supply. Though the trickle current level is typically small, it can cause metal ions from the electrolyte solution to deposit as metal film on the electric contacts, and thereby alter the electric characteristics of the electric contacts. Changes to the electrical or physical characteristics of the electric contacts, including oxidation, crystal formation, and deposition, adversely affect the consistency of the electric voltage/current supplied by the power supply through the electric contacts via the electrolyte solution to the substrate seed layer and adversely affects the resultant uniformity of the deposition metal film.
Therefore, there remains a need for an electro-chemical plating system that limits the current leakage from the power supply through the electric contacts and reduces the metal film deposition onto the electric contacts resulting from the current leakage.